Work, Energy and Power Class 11

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Work Energy and Power Class 11

With exams around the corner, it is always helpful to have notes to refer to. Work, energy, and Power are the most commonly used concepts in Physics Class 11. They are probably the first thing you have learned in your science class. Work and energy are synonymous with each other. And If you want to become a physicist then your basics should be strong. Worry not! In this blog, we will study more about the idea of work, energy, and power in class 11. 

Must Read: Class 11 Gravitation Notes


Let us have a quick overview of some of the important basics of work, energy and power class 11 chapter. Through this, we will understand what is work, energy and power.

Basics of Work

DefinitionWhen a force applied to an object moves that object, then it is called as Work
FormulaW = F × d
SI UnitThe SI unit of work is labelled as joule (J). 

Basics of Energy

DefinitionEnergy is defined as the capacity to do work.
FormulaP.E. = mgh
Si UnitThe SI unit of energy is called as joules (J)

Definition of Power

DefinitionThe rate at which work is done i.e. the energy converted can be defined as Power. 
FormulaP = W/t
SI UnitThe SI unit of power is called watt (W).

Basics of Work  

Let us understand the definition of work in the chapter work, energy and power class 11.

Work is said to be done as the body or entity moves by the use of external force. Work can be interpreted as an action requiring motion and power in the direction of the applied force. 

Formula: W = F × d

For example, if a force of 30 Newtons (N) moves an object 3 meters in the same direction then the force does 90 Joules (J) of work.

The SI unit of work is the joule (J). 


An object is pulled across a surface with a 100 N acting horizontally parallel to the surface. Calculate the amount of work done which is performed by the force in the moving object through a distance of 8 m


Given, F = 100 N
d = 8 m
And since F and d are present in the same direction,
θ = 0, [θ is the angle of the force to the direction of movement]
W = F Cos θ
= 100 x 8 x Cos 0
= 800 J [Since Cos 0 = 1]

Definition of Energy

Next in work, energy, and power class 11, we have the definition of energy.

Energy is defined as the ability to do a task. It can neither be produced nor lost but can only be converted from one type to the next. Therefore, the unit of energy is the same as the unit of work, i.e. Joules. Energy is present in various products thus, the various types of energy are vast. 

Formula: P.E. = mgh

Furthermore, most forms of energy seem to be either kinetic or potential. The energy of action is defined as kinetic energy, while the potential energy is the energy contained in the body and is determined by the amount of work done.

The SI unit of energy is called joules (J).

Kinetic Energy  

Energy generated by the body because of its motion is regarded as the kinetic energy of the body. Kinetic energy acquired by a moving body is equivalent to average work performed by the body just before it comes to rest. 

Kinetic energy = 1/2 mv^2 

Potential Energy  

Energy which is generated by the body as a consequence of its position or state is defined as Potential Energy. Some of the examples for potential energy are energies like gravitational potential energy, electrostatic potential energy, elastic potential energy etc.

Definition of Power

Moving forward in work, energy and power class 11, we will examine the meaning of power. Power is a physical phenomenon that has a variety of different interpretations, depending on the situation and the knowledge accessible. The rate at which work is performed is known as power. This is a measure of energy consumed per unit of time.

Formula: P = W/t
Where, P = Power, W = Work done, T = Time taken

Definition of Work Energy Theorem

The work done on a body by applying force is equal to the change in kinetic energy of the body. This is defined as Work-Energy Theorem. 

Law on the Conservation of Energy 

The Law of Conservation of Energy states that the total energy of an isolated system does not alter or change. However, energy can be converted from one form to another, but the total energy that is present in an independent entity that remains unchanged. 

Energy cannot be produced or Lost

In addition to mechanical energy, it can manifest itself in many other ways. Any of these forms include: thermal energy, electrical energy, chemical energy, visual light energy, nuclear energy, etc. 

Mass and Energy Equivalence 

Next in work, energy, and power class 11, we have the equivalence of mass and energy. Einstein says that energy and mass are synonymous i.e. mass can be transformed into energy, and energy can be transformed into mass.


Collision is characterised as an isolated occurrence in which two or more colliding objects exercise relatively strong forces over a relatively short period of time. The collision between particles has been widely categorised into two forms which can be defined as: 

  • Elastic Collisions
  • Inelastic Collisions

Elastic Collisions 

A collision between two particles or bodies is considered to be elastic when both the linear momentum and the kinetic energy of the system are preserved then it is known as Elastic Collisions. Eg: a collision between atomic particles, electrons, marble balls, and billiard balls.

Inelastic Collisions 

A collision is considered to be an inelastic collision if the linear momentum of the system is retained but, the kinetic energy is not maintained.

Work, Energy and Power Numerical

The following are the practice numericals for work energy and power:

Q1. Find the energy possessed by an object of mass 10kg when it is at a height of 6m above the ground.
Take, g=9.8ms−1

Q2. A stone is projected vertically up to reach maximum height ′h′. The ratio of its kinetic energy to potential energy, at a height 4h/5​ will be?

Q3. A car is accelerated on a levelled road and attains a velocity 4 times of its initial velocity. In this process, the potential energy of the car is?

Q4. A boy weighing 50kg climbs up a vertical height of 100m. Calculate the amount of work done by him. How much potential energy does he gain? (g=9.8m/s2)

Q5. What will be the potential energy of a body of mass 5 kg kept at a height of 10 m?

Example Questions

Here are a few important example questions to make you thorough with the concepts of work, energy, and power in class 11. 

  1. Comets are passing around the sun in extremely elliptical orbits and the gravitational force on the comet due to the sun is not natural to the velocity of the comet in general. Yet, the function of gravitational force over a full orbit of the comet is negligible. Why? 

Ans. The force of gravity is conservative. The work undertaken by conservative forces on a closed path is zero. Thus, for a full orbit of the comet, the work done by the gravitational force is zero.

  1. A molecule with a speed of 300 m s-1 hits the wall of the container at an angle of 40 degrees with the normal and rebounds at the same speed. During the collision is the momentum conserved?

Ans. Whether the collision is an elastic or an inelastic collision, the momentum is conserved. The molecule travels at a speed of 300 m/s and strikes the wall and rebounds with the same speed. Therefore, the rebound velocity is zero. And the kinetic energy is conserved during the collision.

Explore: Class 11 Oscillations Notes

Well, this was all about Work, Energy, and Power  Class 11. We hope that our notes help you and wish you all the best. Check out Leverage Edu for more Class 11 notes.

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